I suspect any competent lawyer would tell you this paragraph is so ambiguous as to be very expensive to litigate with little certainty about the outcome, especially in front of a jury likely to see one large company trying to pick on a competitor by initiating the suit. I suspect Disney has no desire to waste the money required to find out.
Disney is getting paid royalties, and they are most likely gearing Phase 4 toward properties they can use. I suspect they will let Universal hold on to the properties they have. It cuts both ways for Universal, promoting something that Disney owns, paying them some token amount of money (and it is a token since Marvel was desperate when the contract was written), but getting some quality IP for very little.
Basically, don't hold out hope. Legally this would be very difficult and the incentive just isn't there. Maybe someday it will change and Universal will have a property of their own they want to go in with in a big way in that area and they will reasonably sell the rights back to Disney. That's about the best hope. But by the time that happens, Marvel, in the way Disney can use it, will probably already be well established in WDW and it will take a ride generation or two until they need to get rebranded. And by then, who knows if Hulk or Captain America or a Phase IV less popular character will have been made the biggest star. It might never be worth having an Avengers ride in WDW...